Pumps with dual inlet impellers have smaller outer diameter compared to single-inlet impeller pumps due to the higher rotational speed of the pump shaft, making them more preferable for placement inside the reactor casing. However, the addition of the second inlet flow to the impeller leads to a complication in the construction of the flow-routing part of the pump due to the mutual intersection of the exhaust (pressurized) flow with one of the two intake flows to the dual intake impeller. In turn, this leads to an increase in the outer diameter of the pump in order to provide the necessary flow rates for both the outlet and inlet with the aim of achieving maximum pump efficiency. Additionally, attaining high pump efficiency is prevented by inlets (leading to the wheel rim) competing for the same space used by the circular outlet. The rotating pressurized flow intersects the inlet flows at right angles on the diametric plain. The sharp right angle turn causes impact of the outlet flow against the inlet walls with formation of vortices, which create significant hydraulic resistance for the outlet flow between inlets. One part of the outlet flow continues to move between inlets, while the other part keeps rotating in the circular outlet, thereby reducing the pump efficiency.
The increased speed of the dual inlet impeller pump shaft and the reduction of the effective intake flow area due to interference between inlet and outlet flows result in greater relative speed of inlet flow at the point of impeller entry and thus to a greater drop in pressure at the impeller inlet, which lowers anti-cavitation properties and service life of the pump. In order to maintain continuous, cavitation-free pump operation it is necessary to increase the relative gas pressure in the reactor's gas chamber, but it is limited and shouldn't exceed 0.05 MPa due to the constraints of its casing strength based on practical limits for reactor size, quantity of metal used, and sealing assemblies design.
Furthermore, casting elements are used due to the complexity of constructing flowing parts of the dual inlet impeller, lowering the quality of pump manufacturing technologies, which leads to increased wall thickness in the flowing part, mass and size dimensions and higher pump manufacturing costs.
Using for the inlet flow only one annular area between the casing and outer sidewall leads to the mutual influence of profluent axial jets on one another, resulting in the appearance of vortices that penetrate the entrance to the impeller and decrease flow velocities. The large hydraulic resistance at the inlet flows due to the flow bending from the axial direction to a radial one, as well as the limited inlet flow section decrease the pump efficiency, anti-cavitation properties and service life. Besides the use of elements with geometric similarity in pump construction, this fails to achieve dynamic similarity on the operating rim of the impeller and therefore fails to balance the workload of the impeller rims.
High resource demands, required for the reactor as well as its equipment, serve primarily for the development of an improved pump design with dual inlet impeller, specifically of a more compact pump, with a minimum outer diameter that simultaneously accommodates the largest possible impeller diameter inside, working at a reduced rotational speed, and having increased anti-cavitation properties, service life and efficiency.
A known centrifugal pump (see, e.g., cert. of author, USSR No 823653 cl. F04D 29/42 from 23 Apr. 1981), comprising a casing with annular collector and a supply pipe, containing a dual inlet impeller and annular guiding vanes with blades that have internal cavities that form interblade diffusion channels. Corrugated ridges that limit the annular collector, the projections of which form longitudinal channels that report to the diffuser channels, are installed in the casing. The pump fluid is pumped along the supply pipe to the entrance at the lower rim of the impeller. At the same time, the fluid is supplied to the upper rim of the impeller along the internal cavities of the annular guide vane. Having increased the energy in the impeller, the fluid enters the annular collector and further on the discharge pipe via the interblade diffusion channels.
The following are the main disadvantages of this pump design:
The intersection of the inlet and outlet flows in the diametric plane of the pump mutually act upon each other and wedge the flat annular flow area between themselves, which leads to an increase in pump diameter in order to achieve an acceptable pump efficiency.
Inconsistency in geometric similarity of input to the upper and lower impeller rim, a much greater hydraulic resistance, and greater non-uniformity of the velocity field attributable to the upper rim significantly reduce its anti-cavitation properties and as a consequence, its service life.
Minimizing the pressure loss at the exit from the guide vanes, the pump achieves great pressure losses in the form of leaks along the air gaps, upper and lower, in the pump casing, as the gaps come under the pump pressure from the annular collector, which reduces the efficiency of the pump.
A known pump (see, e.g., French patent application No 1246860, cl. F04D1/00; F04D1/06, of Nov. 25, 1960), comprising a cylindrical housing, containing a dual inlet impeller, vane outlet from the pumping chamber, connected to the discharge collector through overflow channels, alternating with radial feed channels in the cross section of the pump.
The main disadvantages of the pump are the same as in the aforementioned design:
The intersection of the outlet and inlet flows in the cross section of the pump does not provide a minimum pump diameter.
Inconsistency in geometric similarity of input to the upper and lower impeller rim. Great hydraulic resistance and greater non-uniformity of the velocity field attributable to the upper rim, and additionally the negative effect of the rotating shaft on the velocity field upon entry to the upper rim significantly reduce its anti-cavitation properties and as a consequence, its service life.
The low efficiency of the pump is due to the large hydraulic resistance at the flow exit from the blade outlet into the overflow channels, positioned at a right angle, where vortices form and fill the outlet section.
The closest analog from prior art to the invention, chosen as a prototype, is the vertical centrifugal pump (see, e.g., article: Kostin V. I., Kuropatov A. I. On the choice of the main circulator pumps for the primary circuit of prospective installations with sodium coolant. Publisher “Energy” magazine “Thermal Engineering”, Mar. 3, 1978, pp. 54-57, FIG. 2.), comprising a cylindrical housing, containing a dual inlet impeller with upper and lower rows of blades, annular guide vane, annular outlet with outer and inner casing forming the a collector in the lower part, which feeds into the discharge pipe, and channels to supply fluid to the lower and upper rows, that latter formed by the housing and outer casing. The fluid being pumped enters the impeller from the annulus formed by the housing and outer casing. At the same time, fluid is supplied to the upper rim through the upper inlet channel, positioned radially and uniformly around the circumference with a flow rotation of 180°, and to the lower rim through lower inlet channel that is geometrically similar to the upper inlet channel and rotated by 45° in order to reduce the influence of axial jet flows on each other. Further, the flows from the upper and lower rims combine into a single flow, which exits the impeller into the annular guide vane and from there into the annular outlet between the inner and outer casings. From the annular outlet, the rotating annular flow experiences a 90° downward bend and an impact against the inlet channel walls and passes to the lower rim between them into the collector, which is formed in the lower part by the inner and outer casing, and further into the discharge pipe connected to them.
The disadvantages of the described pump design are as follows:
Large pump diameter. As a result of the intersection in the center plane of the annular outlet's inlet channels with the lower rim it is impossible to achieve a minimized outer pump diameter. Clogging of the outlet's annular channels by the inlet channels to the lower rim requires a larger area for the outlet's flow section in order to achieve maximum efficiency, which leads to an increased pump diameter. Additionally, the location of the annular guide vane in the center impeller plane between the annular outlet and impeller is not effective and increases the outer diameter of the pump by the width of the annular guide vane. It should be added that the greatest effectiveness of radial-type annular guide vanes is achieved by extending the diffusion channels, leading to an even greater increased in its outer diameter.
Complexity of the pump's flow parts design. In view of the complex geometric form of the inlet channel profile in the pump, casting elements are used, which lower the production quality of the pump's flow parts due to the sinks that form in them that are subsequently refined in the manufacturing process. The use of casting increases thickness of the flow parts walls, size and mass dimensions and the cost of manufacturing the pump.
Low pump efficiency. An imperfection in diverting the flow after the annular guide vane along the passage between the inlet channels reduces the efficiency of the pump. After the guiding vane, the rotating annular pressure flow has a high peripheral speed and when crossing the center plane with inlet channels at 90° suffers significant hydraulic resistance from the impact against the sidewall of the inlet channel in the form of vortex formation, which crowd the section along the vertical passage down between inlet channels and significantly lower pump efficiency. In addition, lower pump efficiency also contributes to significant hydraulic resistance from the inlet to the upper rim due to crowding of the channel-shaped section that replicates the lower rim. The radially positioned inlet channels in the center plane of the pump that supply the lower rim have a significant hydraulic resistance when the flow turns from an axial direction to a radial one. All of this additional hydraulic resistance on the inlets to upper and lower rims is introduced for the sake of geometric similarity upstream from the annular space between the pump housing and the discharge pipe. During pump construction, the use of inlets in the form of separate radial current flows at the impeller entrance does not provide a good velocity field around the entire entrance section upon suction, which reduces its efficiency, anti-cavitation properties and service life.
Lower anti-cavitation properties and service life of the pump. The increased rotational speed of the pump shaft to the dual inlet impeller and the crowding of the section where flow is supplied to the impeller are due to the mutual influence of inlet and outlet channels on each other lead to an increase in the relative velocity of the flow at the inlet to the impeller and consequently to an even greater reduction in pressure on the impeller inlet, which reduces the anti-cavitation properties and service life of the pump. To maintain continuous and cavitation-free operation it is necessary to increase the excess gas pressure in the reactor cavity, which leads to increased stress on the solid housing of the reactor and reduces its service life.
Problems with flow inlets to the impeller. In pump construction, when implementing geometric similarity in the inlet flow to the upper and lower impeller rims, the upper impeller rim is in the worst operating conditions due to the negative effect of the rotating shaft surface on the incoming radial-axial flow. The rotating shaft curls the incoming flow to the upper impeller rim and introduces additional irregularity to the velocity field with the formation of vortices on the inlet rim, which increases the relative velocity of the flow upon impeller suction and reduces its anti-cavitation properties. Furthermore, the upper impeller rim is in the worst operating conditions in relation to the lower rim because the smaller hydrostatic lift of the amount of geometrical differences between markers of the upper and lower rims. As a result of the aforementioned, in addition to the implementation of geometric similarity the dynamic conditions of the rim's velocity field are varied due to the lack of influence of the rotating shaft on the flow to the lower rim, which puts the lower rim in the best conditions for anti-cavitation properties. Rim operation is far from balanced and demands further improvement to pump design. Additionally, the pump uses one annular section between the housing and outer casing of the pump as an axial inlet to the upper and lower rim. In order to reduce the influence of the current of radial inlet flows on one another, the pump utilizes a turn of the upper inlet channels of 45° in relation to the lower channels. However, exclusion of the influence of axial flows on one another through the sides is not achieved, which leads to vortex formation, which extend deep into the pump impeller and distort the velocity field at the impeller inlet.